Convert readthedocs links for their .org -> .io migration for hosted projects

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

CHANGES.rst

@@ -129,7 +129,7 @@ CHANGES
 .. _GitLab#35: https://gitlab.com/pycqa/flake8/issues/35
 .. _GitLab!18: https://gitlab.com/pycqa/flake8/merge_requests/18
 .. _GitLab!20: https://gitlab.com/pycqa/flake8/merge_requests/20
-.. _VCS hooks docs: https://flake8.readthedocs.org/en/latest/vcs.html
+.. _VCS hooks docs: https://flake8.readthedocs.io/en/latest/vcs.html
 
 2.3.0 - 2015-01-04
 ------------------
@@ -222,7 +222,7 @@ CHANGES
 - McCabe complexity warnings are prefixed by a ``C`` instead of a ``W``
 - Flake8 supports extensions through entry points
 - Due to the above support, we **require** setuptools
-- We publish the `documentation <https://flake8.readthedocs.org/>`_
+- We publish the `documentation <https://flake8.readthedocs.io/>`_
 - Fixes #13: pep8, pyflakes and mccabe become external dependencies
 - Split run.py into main.py, engine.py and hooks.py for better logic
 - Expose our parser for our users

README.rst

@@ -92,6 +92,6 @@ suggest, the mailing list would be the best place for it.
 Links
 =====
 
-* `flake8 documentation <http://flake8.readthedocs.org/en/latest/>`_
+* `flake8 documentation <https://flake8.readthedocs.io/en/latest/>`_
 
-* `pep8 documentation <http://pep8.readthedocs.org/en/latest/>`_
+* `pep8 documentation <https://pep8.readthedocs.io/en/latest/>`_